The invention is in the optoelectronic field. The invention is applicable to optical modulation systems, including, for example, optical switching for digital signaling and small signal modulation for analog applications.
Optical modulators are used in a variety of systems. Controlled modulation of laser light is useful in analog systems to produce an output proportional to the input signal. Digital optical systems, such as fiber optic communication systems, use optical modulators to signal digital signals. In such case, a modulator is controlled to turn on and off. Digital optical modulators as signaling devices may also form the basis for optical memories and general computer devices. Possibilities for optical modulators in both digital and analog systems are increased with increased efficiency as measured with respect to the drive voltage required to produce the desired optical modulation.
Conventional modulators follow similar radio frequency transmission theory of attaining the desired transmission or reflection over the width of the pass band, typically attempting to use the center portion of the pass band or bands of the modulator. Conventional modulation of optical waves utilizes the change of the refractive index and/or the change of the absorption coefficient as a function of applied voltage to modulate the intensity or phase of an optical wave. Example conventional devices operating in this manner over their pass bands are the Mach-Zehnder modulator, the electro-optical phase modulator, the semiconductor electro-refraction modulator, and the electro-absorption modulator. Any of these modulators would be rendered more useful by an increased efficiency as a function of drive voltage.
The method of the invention uses the band edge of periodic structures for an efficient modulation method. In the band edge region, transmission and reflection of an optical wave can be made very sensitive to the change in radiation wavelength, a change in the modulator material refractive index, and/or a change in the material absorption. Controlling these parameters with the increased level of sensitivity is provided by modulation using the band edge region.
A preferred embodiment method of the invention uses a periodic optical structure (i.e., a grating) on top of an optical waveguide structure. The combination of the periodic structure and the optical waveguide is designed so that the reflection and/or transmission of the guided wave have broad pass bands with narrow transition bands. The optical structure is exposed to an incident laser radiation with a wavelength in one of the transition bands. Modulation of the incident laser radiation is controlled by the change in refractive index or absorption in the optical guided wave structure produced by the modulation voltage. The incident radiation is in the transition band, instead of the center portion of the pass band.
A preferred method for designing a suitable periodic optical structure having broad pass bands with narrow transition bands uses a high coupling coefficient between the periodic structure and the optical waveguide mode. The larger the product of the coupling coefficient and the length of the grating, the narrower the transition region band, and the more sensitive the modulation. The periodicity is chosen (by fabrication and by applying a bias voltage) so that the wavelength for Bragg reflection of the periodic structure is offset from (i.e., mismatched to) the incident laser wavelength so that the laser wavelength is at the center of one of the transition bands. A suitable method for designing the combined structure is to vary various dimensions and materials, including grating, waveguide, and electrode, so that there is a large electro-optic change of index or absorption, a strong coupling coefficient and length product, a low insertion loss, and sufficiently fast speed of operation for the intended application.